Field of the Invention
The present invention relates to a machine tool.
Discussion of the Background
Machining equipment (a machine tool), such as a CNC turning center, typically incorporates a single integral spindle motor in a spindle housing for rotating the spindle, which holds a workpiece. Because such a motor drives the spindle directly, a gear transmission is not required. The drive mechanism by the motor will turn the spindle at high speeds for a turning operation, such as removing material to reduce the outside diameter of the workpiece. The same motor may also be used for low speed, precisely controlled angular rotation of the spindle for C-axis contouring or milling However, for larger diameter workpieces, such as those having a larger spindle bore, the motor may not be available or economically feasible to produce. For a turning center to machine such workpieces, a separate motor with a gear transmission may be used to rotate the spindle. However, such a drive configuration with the gear transmission is not well suited for machining operations that require high accuracy, such as C-axis contouring, because accuracy to position the rotational angle of the spindle is lost due to backlash or play in the gear transmission.
Driving a turntable for supporting a workpiece to be subjected to machining, such as that of a vertical machining center, with two separate motors is known in the art. For example, a turntable may be driven with one motor for high and medium speed operations, and a servo motor for low speed or indexing operations. In such a system, both the motors are connected to the rotor via gear transmissions. Such a drive configuration is not well-suited for contouring applications due to the inaccuracies introduced by the gear transmissions. It is noted that, for example, Japanese Unexamined Patent Application Publication No. S63-191549 and Japanese Unexamined Patent Application Publication No. 2003-170334 each disclose the conventional art concerning the invention.